A number of somatic cells are known to possess receptors for C1q, a subcomponent of the first component of complement, C1. Although the existence and the ubiquity of this receptor (C1qR) has been well established, information concerning its structure and function is lacking. The aim of this proposal is therefore to characterize the (C1qR) with regards to: I. structure, II. biosynthesis, III. distribution, and IV. biologic function using two monoclonal antibodies, II1/D1 and II1/B5 and the cultured cell lines; Raji, Daudi, Wil2WT, U937 and Molt4 as well as peripheral blood leukocytes. The structure (I) of C1qR will be investigated by: 1) SDS-polyacrylamide gel electrophoresis and two dimensional isoelectric focusing of purified or immunoprecipitated C1qR from surface-labeled, detergent-solubilized membranes. 2) Affinity cross-linking and photoaffinity labelin using both, disulfide cleavable and noncleavable heterobifunctional cross-linkers. 3) Labeling C1qR bearing cells with different probes to determine, phosphorylation, glycosylation as well as the existence of reactive thiol groups. 4) Enzymatic digestion of C1qR in the preesence or absence of cross-linkers to assess receptor conformation. The biosynthesis (II) of C1qR will be studied by: 1) metabolic labeling with tritiated essential amino acids. 2) Pulse-chase labeling in the presence or absence of specific enzymes (e.g. Engoglycosidase H, Proteinase K) to determine maturation and structure. 3) "Down" or "up"-regulation with C1q or MoAbs. The distribution (III) of C1qR in cultured cells and peripheral blood leukocytes will be investigated by: 1) Radioligand binding studies to determine the number of receptor sites. 2) Analysis by a fluorescence activated cell sorter to identify subpopulations of cells expressing different ligand affinities or receptor numbers. Finally the biologic function(s) (IV) of C1qR will be examined by: 1) Studying the effect of C1q or MoAb on culture-cell proliferation. 2) Thymocyte proliferation to understand the C1q-mediated inhibition of produciton of "IL-1"-like substance by Raji cells. 3) Phagocytosis and immune clearance of C1q-bearing target cells or immun-complexes. From preliminary studies it has become apparent that Raji or Daudi cells constitutively produce "IL-1"-like substance and C1q can inhibit this production in a dose-dependent manner. This novel observation is important since it suggests that C1qR may play a role in immune regulation. The long-term objectives of this proposal are therefore to understand the biologic significance of such a widely-distributed receptor and to obtain detailed structural information which will be important for understanding the mechanism of its function.